Add comment for visiting children of select in ApplyWithGlobalVisitor

src/Interpreters/ApplyWithGlobalVisitor.cpp

@@ -96,6 +96,16 @@ void ApplyWithGlobalVisitor::visit(ASTPtr & ast)
             }
         }
 
+        /*
+         * We need to visit all children recursively because the WITH statement may appear in the subquery at the nested level.
+         * Behavior of `WITH ... UNION ALL ...` should be the same at the top level and inside the subquery.
+         *
+         * For example:
+         * SELECT * FROM (WITH (SELECT ... ) AS t SELECT ... UNION ALL SELECT ...)
+         *                ^^^^^^^^^^^^     should be visited     ^^^^^^^^^^^^^^^^
+         * or inside `WHERE .. IN` clause:
+         * SELECT * FROM ... WHERE x IN (WITH (SELECT ... ) AS t SELECT ... UNION ALL SELECT ...)
+         */
         for (auto & child : node_union->list_of_selects->children)
             visit(child);
     }